The present invention relates to an exhaust gas purifying system for an internal combustion engine.
It is well known to have an exhaust system of an internal combustion engine equipped with a catalytic converter containing an oxidation catalyst to purify engine exhaust gases. It has also been a common practice to supply air upstream of the catalytic converter for promoting oxidation of unburned constituents of engine exhaust gases. During oxidation of unburned constituents in the catalytic converter, the catalyst is frequently exposed to unduly high temperature, and requires freqent replacement.
In order to overcome this shortcoming encountered in the prior art, it has heretofore been proposed to temporarily interrupt the supply of air into the catalytic converter to control the reaction between the oxidizable constituents of the exhaust gases and the oxygen in the air to prevent an excessive rise in the operating temperature of the catalytic converter. In this prior art, cooling of the catalytic converter is achieved naturally, and the catalytic converter is not cooled down to a desired temperature within a short period of time.
In addition, during interruption of the supply of air into the catalytic converter, gases are purified insufficiently in the catalytic converter because of insufficient oxygen for the reaction with the oxidizable constituents of the engine exhaust gases.